Filler retention in paper making by addition of carboxyalkyl starch ether



United States Patent ice We This invention relates to the production of filled paper.

More articularly, it relates to a method for increasin a the retention of filler in paper while simultaneously retaining and, in fact, improving the strength characteristics of the paper.

In the ma ufacture of the paper web, pulp or Waste paper or both is first dispersed in Water under conditions to minimize any changes in the cellulose fibers. Then the dispersed pulp is subjected to a process of beating or refining in which the pulp is circulated between relatively rotating members that may have bars or knives. Fiber surface and flexibility are increased and fiber length is decreased. Water is taken up by the fibers, and their physical structure is mechanically loosened. There is a decrease in pulp freeness.

Particularly for printing, for ledger and for records paper, it has become necessary to add fillers or pigments to the stock to impart opacity. Paper that is thinner and lighter in Weight is increasingly demanded to reduce the Weight of printed material that must be shipped and mailed. To impart opacity to such papers, the paper is filled or loaded with mineral pigments, particularly clays, calcium carbonate, talc, titanium dioxide, gypsum, blanc fixe, zinc pigments including lithopone. In addition to their ability to enhance opacity, the fillers improve brightness, printability and smoothness. They may also be used to control ink receptivity to some extent.

The use of fillers for these purposes leads to additional problems for the paper manufacturer. The addition of such non-fibrous materials in substantial proportion makes the paper web Weaker both in wet strength during its formation and in dry strength. To state it simply, filled paper has less strength than equivalent paper containing no filler. it has been estimated, for example, that the use of clay results in about a loss in burst strength and much higher loss in folding endurance.

In-the normal operation of the Fourdrinier machine, the stock or furnish containing the mineral pigment filler is fed from a headbox onto the who where the Wet Web is formed. Water is drained from the stock first by gravity, then by suction and finally by pressing. The white Water removed from the stock is normally recirculated and for economy, particularly when pigments are used, it must be recirculated.

in making filled paper, it is necessary to maintain a sutiicient concentration of filler in the headbox to assure that the web retains the desired amount of pigment. This leads to several additional problems. if retention of filler in the paper web is low, the headbox must have a high concentration of filler. This, in turn, decreases the rate of drainage from the wire and, in general, impedes production. Furthermore, the high filler content in the headhoX and its associated circulation systems produce a relatively high rate of erosion and tire high rate of filler circulation also decreases wire life.

In addition, when large amounts of filler are being circulated in the White Water system, the problem of claritying the white Water is compounded. It is necessary to recover the pigments efficiently for economy of operation. Further, the eilluents from the white Water clarification system require additional treatment to reduce stream pollution.

It is evident then, that it is highly desirable that the paper web be formed in such manner as to retain as high a proportion of the filler in the stock as possible, for in this way the amount of filler circulating in the headbox and in the white Water can be reduced.

It is one object of this invention to provide a method of increasing the retention of filler as the Wet paper Web is formed.

In many of its uses, paper is relied upon because of its structural characteristics. Itis required to have burst strength, folding endurance, tensile strength and resistance to tearing. These are in addition to the properties valued for printing or Writing such as opacity, brightness, ink receptivity and so on. The structural characteristics of the paper are generally attributed to the continuous, randomly oriented, three-dimensional network of fibers. It is common in making paper to add to the stock any of a wide variety of non-cellulosic polysaccharides in order to improve the paper in various ways. Among such polysaccharides are starch, vegetable gums, cellulose ethers, etc. An almost endless variety of starch products ranging from unmodified starch through the ordinary modified starches such as oxidized and acid-hydrolyzed varieties to starch others and esters of almost all known types. See, for example, Cushing and Schumann, TAPPI, vol. 42, 1606 to 1016 (1959) and Swanson, TAPPT, vol. 44, 142A to 181A (1961). Almost any of the varieties of starch increase the burst strength and tensile strength of the paper, but some are better than others. It is thought that starch serves as a crosslinking agent through a hydrogen-bonding mechanism, that is, some sort of celluloseto-starch-to-cellulose complex is formed to increase the strength of the bonds in the paper. The starch performs this function since it is in the pasted or highly dispersed form and can therefore achieved contact with the fibers whereas normally, some individual fibers may not be close enough to form a bond. In a sense, starch serves as an adhesive for this purpose.

In the presence of a substantial proportion of filler, however, starch loses much of its etficacy for this purpose. As noted before, the filler introduces inhomogeneity into the Web. It also tends to disrupt the bonds formed between the cellulose and the starch. Of course, it also dilutes the starch. In fact, it has been observed that many varieties of starch that are used to strengthen unfilled papers actually appear to have the reverse ciiect in the presence of the filler or they decrease the retention of the filler.

in recent years, there have been introduced several derivatives of starch which are designed to increase the retention of filler in paper and at the same time to improve the structural p-roperties of filled paper. These starch derivatives are usually ethers that have a cationic group, especially an amine or quaternary ammonium group. See, for example, Patents Nos. 2,935,436 and 3,017,294. It has been assumed that the cationic charge is necessary to flocculate fillers which normally have an anionic charge. In fact, there are other patents, proceeding on the same theory, that suggest the use of wholly synthetic polymers that are cationic, for example, polyacrylamide, melamine-formaldehyde and ureaformaldehyde condensation products and many others. See Patents Nos. 2,838,397; 3,021,257 and 3,019,156, for example. All of these proceed on the same assumption, namely that a cationic polymer is necessary to increase filler retention in the paper Web While improving the structural properties of the paper.

it is another object of this invention to accomplish the same purpose through the use of a starch derivative that is anionic. Other objects of the invention are in part obvious and in part pointed out in the following description.

It has been discovered that by adding the sodium salt of a carboxy alkyl ether of starch to the paper stock, downstream of the beater, the stock containing rosin size and a polyvalent metal ion, and by maintaining in the paper stock a pH in the range of 4.5 to 6.5 the paper produced retains filler to a very high level and simultaneously its structural properties are improved even though it contains a relatively high portion of filler. In addition, this method is highly advantageous in the recovery of pigment and fiber fines from the white water. The first step in the method of this invention is to prepare a stock containing the cellulose and the desired amount of filler, and this is done in the normal way. Then rosin size is added.

The purpose of sizing paper is to control the penetration of liquids, especially inks and water, into the paper. It has been found, in accordance with this invention, that rosin size is not only compatible with the present method, it cooperates with the other ingredients in imparting the desired characteristics of the paper. For this purpose, the rosin size is used in a concentration of 0.5 to 5.0%, the amount varying, as desired, with the type of paper being prepared. Book paper will have a relatively low rosin content, 0.5 to 2.0%; ledger paper 1.5 to 2.5%; bond paper 1.0 to 2.0%; and blueprint paper 2.5 to 5.0%, all by weight of the dry fiber. The rosin is added to the furnish, or stock, before the starch is added since it has been found that the results are relatively poor if the reverse procedure is used.

After the rosin has been added to the furnish, alum, preferably paper makers alum, is added to precipitate the size on the fibers. The proportion of alum used is 1.0 to 3.0 parts per part of rosin by weight. For maximum filler retention, the pH of the furnish must be maintained in the range of 4.5 to 6.5. Normally, it will be found that alum sufiicient to set the size properly will establish a pH in this range. Instead of alum, it has been found, other salts of trivalent metals can also be used. For example, on an equivalent basis, one may use ferric sulfate, aluminum chloride, ferric chloride, and so on. Although the pH range of 4.5 to 6.5 is critical for maximum filler retention, it has been found that up to a pH of about 8, filler is retained to about the same extent as that obtained in the absence of the starch ether of this invention; at the same time the strength of the paper is further improved. Also, better results are obtained when titanium dioxide is used as the filler than are obtained when clay is used.

The proportion of filler used is in the range of 2 to 40% of the fiber depending on the type of paper, and normally is in the range of 3 to 20%. The filler, for best results, should be in the stock before the alum is added. It is evident, that when an acid-sensitive pigment filler such as calcium carbonate is used, the pH should be maintained at the higher end of the range of 4.5 to 8.

The preferred fillers are calcium carbonate, titanium dioxide, zinc sulfide, blanc fixe, talc and clay. One surprising elfect of the present invention is its efiicacy in increasing the retention of titanium dioxdie since, it has been reported, titanium dioxide is difiicult to retain when 'rosin size and alum are used.

After the addition of alum, there is added a sodium salt of carboxy alkyl ether of starch, in which the alkyl group has no more than two carbon atoms. The starch is pasted before addition to the paper stock and is added in an amount from 0.1 to 2.0% of the dry weight of cellulose. It has been found, as illustrated hereafter, that this proportion is critical because the retention of filler is reduced by amounts outside this range. In this process, the normal amount of cellulose fiber is used, i.e., 0.5 to 5%.

The carboxy alkyl starch ethers used in this invention are made by known methods to produce the ethers in the granule form. The carboxy ethyl ether of starch is prepared by the process of application Serial No. 817,036, filed June 1, 1959, for Taylor, now US. Patent No.

3,071,573. In this process, granular starch is reacted in the presence of a limited quantity of alkali with an acrylate ester such as ethyl acrylate. The starch ether forms at the terminal vinyl group, and the alcohol moiety is removed by saponification, yielding the alkali salt of the carboxy ethyl ether of starch. The carboxy methyl starch ether is prepared conveniently by the method of Kesler et a1. Patent 2,773,057. In this process, granule starch is reacted with sodium chloracetate or chloracetic acid in the presence of a limited quantity of alkali. The starch ether forms by the elimination of the chlorine, yielding the alkali salt of the carboxy methyl ether of starch.

After the addition of the starch ether, the pH of the furnish should be checked to make certain that it is within the proper range of 4.5 to 8. Adjustments in the pH of the stock may be made at this point by adding any of the commonly used strong acids, e.g., mineral acids such as hydrochloric or sulfuric acid, organic acids such as acetic or benzene sulfonic acid. When necessary to increase the pH to this range, any of the cornmonly used alkalis, e.g., alkali hydroxides such as sodium or potassium hydroxide, sodium carbonate, ammonium hydroxide and so on, can be used. The purpose of such pH adjustments is to take account of the relative acidity or alkalinity of the water used. The critical matter is the pH of the stock as it is laid on the wire.

Following the addition of the starch and any adjustment in pH, the pulp is sheeted and dried in normal fashion.

Carboxy alkyl ethers of starch have been suggested as useful in sizing paper, i.e., as a beater additive, for example, or for strengthening'paper. For example, in the Cushing et al. article referred to before, it is noted that carboxy ethyl ether of starch tends to strengthen the paper web. It was likewise noted that oxidized starch '(Stayco M) which contains a substantial proportion of carboxyl groups, also increases the strength of the web, and to about the same extent. However, oxidized starch, it has been found, not only does not increase the retention of filler, but it actually decreases it when compared with a blank using no filler retention aid. It was therefore surprising that the carboxy alkyl group, having only one or two carbon atoms between the carboxyl group and the starch molecule, would increase the retention of filler and at the same time retain the strength improvements gained.

The following examples illustrate the method of this invention, it being understood that the invention is not to be limited by the details disclosed.

EXAMPLE I A suspension of bleached sulfite fibers was beaten to a Schopper-Riegler freeness of 620 ml. The pulp was sized with 2% rosin size based on the fiber weight. To the fiber suspension, there was added 10.6% titanium dioxide pigment based on the total fiber and pigment Weight. The pH was adjusted to 5.05.5 with a solution of paper makers alum and then 0.5% carboxyl ethyl ether of native corn starch, in the form of an aqueous paste, was added to the stock. Three replicate sheets were prepared from this stock using a Noble and Wood laboratory sheet machine. The sheets were pressed and dried on the dryer with the temperature about F. for a period of three minutes. A blank was prepared in the same way omitting the pasted starch.

The filler retention of the experimental samples was measured by determining the ash content on ignition using the usual laboratory procedure. For the blank, the ash content was 5.12% and the average ash content of the samples containing the carboxy ethyl starch ether was on an average of 7.55%, an increase of 47.5%.

5 EXAMPLE II The method of Example I was repeated in allessential details except that the fibers were beaten to a freeness of 670 and adjusted to a pH of 5.1. The filler content was 10% titanium dioxide based on the fiber weight. Three blank samples were prepared, as described, and a series of samples containing the carboxy ethyl ether of native corn starch was prepared, varying the proportion of the tarch ether. The results are set 'forth in Table I, the burst strength being determined with a Mullen Tester.

Table 1 Percent Percent Percent Increase Burst Starch Ash Ash Strength,

Ether Over psi.

Blank This example illustrates the effect of pH on the retention of filler. In this case, the proportion of carboxy ethyl starch ether (native cornstarch base) was maintained constant at 0.5% by weight of the fiber and 13 /2% filler clay was used, based on the weight of fiber, in place of the titanium dioxide. Otherwise, the pro cedure was the same as that previously used. The results are set forth in Table II.

Table 11 Percent Starch Used pH Percent Increase Ash Ash Ovcr Blank 4. 6 5. 81 4. 6 8. 79 51. 3 4. 9 5. 62 5. O 7. 83 39. 3 5. 4 5. 59 5. 5 9. 33 67. 0 5. 9 5. 55 5. 9 9.11 64. 0 7. 5 4. 58 7. 5 5. 11 ll. 5

1 O.E.S.=Carhoxy ethyl starch ether.

EXAMPLE IV This example illustrates the effect of the base starch used in preparing the carboxy ethyl starch ether and indicates that the kind of starch is of little consequence so long as native starch is used that is not degraded by hydrolysis or oxidation. The procedure followed was essentially the same as Example II except that the proportion of carboxy ethyl starch ether was 0.5 throughout for each of the samples. The results are indicated in Table III. It is notable that even with those native starch varieties lrnown to increase filler retention, e.g., root and waxy starches, there is a substantial further increase when the corresponding carboxy ethyl derivative is used.

X O.E.S.-Carboxy ethyl ether of native starch indicated.

EXAMPLE V In a larger trial of this invention, using about the same proportions of rosin and alum as in the previous examples, there was prepared a substantial quantity of an -lb., 25 x 38 antique ollset sheet loaded with 200 parts by weight clay and parts by weight titanium dioxide per 2600 lbs, of fiber. The results obtained were compared with those obtained at the same proportion, 0.5 but using a cationic starch which is a commercially available tertiary amine alkyl ether of starch. The retention of filler and the strength characteristics of the paper produced are set forth in Table IV.

EXAMPLE VI The method of Example V was repeated to prepare a 90 1b., 25 x 38 cover stock and the results are set forth in Table V.

Table V Anionic Cationic Percent Ash 11.1 11.2 Mullen (burst strength, p.s.i.) 15. 7 44. 9 MIT Folding Endurance MD/OD 280/88 200/62 IGT Printability, W/F.#6 Ink B Spring 380/255 300/ Dennison Wax Pick, W/F 18/14 16/14 EXAMPLE VII The procedure of Example V was repeated to prepare a 70 lb. bulky otiset grade. In this case, the proportion of carboxy alkyl starch ether (based on native cornstarch) was increased 0.25%. The results are set forth EXAMPLE VIII In order to determine the effect of carboxyl groups of the retention of filler, oxidized corn starch and crosslinked oxidized corn starch were used in preparing paper sheets according to the method of Example I, replacing the carboxy alkyl ether. It was found that even at 1% by weight of these carboxyl containing starch products, an amount useful for strengthening the paper web, the filler retention was reduced from 4.78%, in one case by 40.4% and in the other case by 30.0%.

EXAMPLE IX The method of Example I was repeated in all essential details except that the starch used was the carboxy methyl ether of native corn starch, prepared by the method of the Kesler et al. patent referred to and the starch ether was used at 0.25% of the fiber weight. For the blank, the ash content was 6.11% and for the sample in accordance'with this invention, it was 7.99%.

EXAMPLE X The method of Example I was repeated except that different metal salts were used and rosin size was omitted. The metal salts were used at a concentration of 1% or" the fiber weight. With the blank result at 4.73%, the ash content when the salts were used was: for aluminum chloride 8.28%, for ferric chloride, 7.72%, and for aluminum sulfate 6.52%.

Since many embodiments of this invention may be made and since many changes may be made in the embodiments described, it is to be understood that the invention is not limited to the details disclosed but is defined by the claims set forth hereafter.

We claim:

1. A process of manufacturing paper to enhance filler retention and burst strength that comprises providing a dilute aqueous suspension of refined cellulose fibers sized with 0.5 to 5.0% rosin based on fiber weight, said suspension containing from 3 to 20% of a mineral filler, incorporating in said suspension pasted carboxy alkyl starch ether in which the alkyl group contains not more than two carbon atoms, the starch proportion being 0.1 to 2.0% of the cellulose fibers by weight, adding alum to said suspension in a ratio of 1 to 3 parts per part of rosin size, sheeting the fibers and filler at a pH in the range of 4.5 to 8 to form a web and drying the web.

2(The method of claim 1 in which the fibers and fillers are sheeted at a pH in the range of 4.5 to 6.5.

3. The method of claim 2 in which the alkyl group is ethyl.

4. The method of claim 2 in which the alkyl group is methyl.

5. The method of claim 2 in which the filler comprises 10 titanium dioxide.

6. A process of manufacturing paper to enhance filler retention and burst strength that comprises providing a dilute aqueous suspension of refined cellulose fibers, said suspension containing from 3 to 20% of a mineral filler, incorporating in said suspension pasted carboxy alkyl starch ether in which the alkyl group contains not more than two carbon atoms, the starch proportion being 0.1 to 2.0% of the cellulose fibers, adding a water-soluble inorganic salt of a trivalent metal to suspension in an amount from 0.5 to 15% of the fiber weight, sheeting the fibers and filler at a pH in the range of 4.5 to 6.5 to form a web and drying the web.

References Cited in the file of this patent UNITED STATES PATENTS 2,113,034 Rowland Apr. 5, 1938 2,195,600 Reilly Apr. 2, 1940 2,680,072 Marrone June 1, 1954 2,773,057 Kesler Dec. 4, 1956 FOREIGN PATENTS 564,585 Great Britain Oct. 4, 1944 OTHER REFERENCES Cushing: Some Recent Advances in Wet-End Addi tives and Implications for Future Research, TAPPI, vol. 44, No. 3, March 1961, pp. 191A-194A. 

1. A PROCESS OF MANUFACTURING PAPER TO ENHANCE FILLER RETENTION AND BURST STRENGTH THAT COMPRISES PROVIDING A DILUTE AQUEOUS SUSPENSION OF REFINED CELLULOSE FIBERS SIZED WITH 0.5 TO 5.0% ROSIN BASED ON FIBER WEIGHT, SAID SUSPENSION CONTAINING FROM 3 TO 20% OF A MINERAL FILLER, INCORPORATING IN SAID SUSPENSION PASTED CARBOXY ALKYL STARCH ETHER IN WHICH THE ALKYL GROUP CONTAINS NOT MORE THAN TWO CARBON ATOMS, THE STARCH PROPORTION BEING 0.1 TO 2.0% OF THE CELLULOSE FIBERS BY WEIGHT, ADDING ALUM TO SAID SUSPENSION IN A RATIO OF 1 TO 3 PARTS PER PART OF ROSIN SIZE, SHEETING THE FIBERS AND FILLER AT A PH IN THE RANGE OF 4.5 TO 8 TO FORM A WEB AND DRYING THE WEB. 